A known immunoassay for detecting an antibody or antigen in a sample is performed using particles on which an antigen or antibody capable of biding to the antibody or antigen to be detected is immobilized. In this method, the particles are brought into contact with the antibody or antigen to be detected so that an antigen-antibody complex is formed on the particles. The antigen-antibody complex formed on the particles is labeled with a marker, which makes it possible to detect the target antibody or antigen.
In general, the above immunoassay is performed using a reagent in which particles having an antigen or antibody immobilized thereon are suspended. In some cases, such suspended particles may aggregate in the reagent. If such a particle aggregate-containing reagent is used in the measurement, the detection accuracy may be reduced. Therefore, particles need to be dispersed sufficiently in the reagent to be used.
It is also known that a certain compound is applied or adsorbed to the surface of particles so that the dispersibility of the particles suspended in a reagent can be improved. For example, Japanese Patent Application Laid-Open (JP-A) No. 2006-282582 discloses a method of coating particle surfaces with citric acid by chemical modification to improve the dispersibility of the particles. JP-A No. 2007-169209 also discloses a method for improving the dispersibility of particles, which includes adsorbing, to the particle surface, a given amount of a dibasic organic acid compound and/or a tribasic organic acid compound. However, the methods disclosed in JP-A Nos. 2006-282582 and 2007-169209 have the problem that the particle surface treatment is complicated and expensive.